array.reduce

The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.

var sum = [0, 1, 2, 3].reduce(function(acc, val) {
  return acc + val;
}, 0);
// sum is 6

var list1 = [[0, 1], [2, 3], [4, 5]];
var list2 = [0, [1, [2, [3, [4, [5]]]]]];

const flatten = arr => arr.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatten(val) : val), []);
flatten(list1); // returns [0, 1, 2, 3, 4, 5]
flatten(list2); // returns [0, 1, 2, 3, 4, 5]

Syntax

arr.reduce(callback, [initialValue])

Parameters

callback

Function to execute on each value in the array, taking four arguments:

accumulator

The accumulated value previously returned in the last invocation of the callback, or initialValue, if supplied. (See below.)

currentValue

The current element being processed in the array.

currentIndex

The index of the current element being processed in the array. Starts at index 0, if an initialValue is provided, and at index 1 otherwise.

array

The array reduce was called upon.

initialValue

Optional. Value to use as the first argument to the first call of the callback.

Return value

The value that results from the reduction.

Description

reduce executes the callback function once for each element present in the array, excluding holes in the array, receiving four arguments:

• accumulator
• currentValue
• currentIndex
• array

The first time the callback is called, accumulator and currentValue can be one of two values. If initialValue is provided in the call to reduce, then accumulator will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was provided, then accumulator will be equal to the first value in the array and currentValue will be equal to the second.

Note: If initialValue isn't provided, reduce will execute the callback function starting at index 1, skipping the first index. If initialValue is provided, it will start at index 0.

If the array is empty and no initialValue was provided, TypeError would be thrown. If the array has only one element (regardless of position) and no initialValue was provided, or if initialValue is provided but the array is empty, the solo value would be returned without calling callback.

It is usually safer to provide an initial value because there are three possible outputs without initialValue, as shown in the following example.

var maxCallback = ( pre, cur ) => Math.max( pre.x, cur.x );
var maxCallback2 = ( max, cur ) => Math.max( max, cur );

// reduce() without initialValue
[ { x: 22 }, { x: 42 } ].reduce( maxCallback ); // 42
[ { x: 22 }            ].reduce( maxCallback ); // { x: 22 }
[                      ].reduce( maxCallback ); // TypeError

// map/reduce; better solution, also works for empty arrays
[ { x: 22 }, { x: 42 } ].map( el => el.x )
                        .reduce( maxCallback2, -Infinity );

How reduce works

Suppose the following use of reduce occurred:

[0, 1, 2, 3, 4].reduce(function(accumulator, currentValue, currentIndex, array) {
  return accumulator + currentValue;
});

The callback would be invoked four times, with the arguments and return values in each call being as follows:

callback	accumulator	currentValue	currentIndex	array	return value
first call	0	1	1	[0, 1, 2, 3, 4]	1
second call	1	2	2	[0, 1, 2, 3, 4]	3
third call	3	3	3	[0, 1, 2, 3, 4]	6
fourth call	6	4	4	[0, 1, 2, 3, 4]	10

The value returned by reduce would be that of the last callback invocation (10).

You can also provide an Arrow Function in lieu of a full function. The code below will produce the same output as the code in the block above:

[0, 1, 2, 3, 4].reduce( (prev, curr) => prev + curr );

If you were to provide an initial value as the second argument to reduce, the result would look like this:

[0, 1, 2, 3, 4].reduce( (accumulator, currentValue, currentIndex, array) => {
  return accumulator + currentValue;
}, 10);

callback	accumulator	currentValue	currentIndex	array	return value
first call	10	0	0	[0, 1, 2, 3, 4]	10
second call	10	1	1	[0, 1, 2, 3, 4]	11
third call	11	2	2	[0, 1, 2, 3, 4]	13
fourth call	13	3	3	[0, 1, 2, 3, 4]	16
fifth call	16	4	4	[0, 1, 2, 3, 4]	20

The final call return value (20) is returned as a result of the reduce function

Examples

Sum all the values of an array

var sum = [0, 1, 2, 3].reduce(function(a, b) {
  return a + b;
}, 0);
// sum is 6

Alternatively, written with an arrow function:

var total = [ 0, 1, 2, 3 ].reduce( ( acc, cur ) => acc + cur, 0 );

Flatten an array of arrays

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(function(a, b) {
  return a.concat(b);
}, []);
// flattened is [0, 1, 2, 3, 4, 5]

Alternatively, written with an arrow function:

var flattened = [[0, 1], [2, 3], [4, 5]].reduce( ( acc, cur ) => acc.concat(cur), [] );

Counting instances of values in an object

var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

var countedNames = names.reduce(function(allNames, name) { 
  if (name in allNames) {
    allNames[name]++;
  }
  else {
    allNames[name] = 1;
  }
  return allNames;
}, {});
// countedNames is { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

Bonding arrays contained in the array of objects using spread operator and initialValue

// friends - an array of objects 
// where object field "books" - list of favorite books 
var friends = [ 
{ name: 'Anna', books: ['Bible', 'Harry Potter'], age: 21 }, 
{ name: 'Bob', books: ['War and peace', 'Romeo and Juliet'], age: 26 },
{ name: 'Alice', books: ['The Lord of the Rings', 'The Shining'], age: 18 }
]

// allbooks - list which will contain all friends books +  
// additional list contained in initialValue
var allbooks = friends.reduce(function(prev, curr) {
  return [...prev, ...curr.books];
}, ['Alphabet']);

// allbooks = ['Alphabet', 'Bible', 'Harry Potter', 'War and peace', 
// 'Romeo and Juliet', 'The Lord of the Rings', 'The Shining']

Polyfill

// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: http://es5.github.io/#x15.4.4.21
// https://tc39.github.io/ecma262/#sec-array.prototype.reduce
if (!Array.prototype.reduce) {
  Object.defineProperty(Array.prototype, 'reduce', {
    value: function(callback /*, initialValue*/) {
      if (this === null) {
        throw new TypeError('Array.prototype.reduce called on null or undefined');
      }
      if (typeof callback !== 'function') {
        throw new TypeError(callback + ' is not a function');
      }

      // 1. Let O be ? ToObject(this value).
      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0; 

      // Steps 3, 4, 5, 6, 7      
      var k = 0; 
      var value;

      if (arguments.length == 2) {
        value = arguments[1];
      } else {
        while (k < len && !(k in o)) {
          k++; 
        }

        // 3. If len is 0 and initialValue is not present, throw a TypeError exception.
        if (k >= len) {
          throw new TypeError('Reduce of empty array with no initial value');
        }
        value = o[k++];
      }

      // 8. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kPresent be ? HasProperty(O, Pk).
        // c. If kPresent is true, then
        //    i. Let kValue be ? Get(O, Pk).
        //    ii. Let accumulator be ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).
        if (k in o) {
          value = callback(value, o[k], k, o);
        }

        // d. Increase k by 1.      
        k++;
      }

      // 9. Return accumulator.
      return value;
    }
  });
}

If you need to support truly obsolete JavaScript engines that don't support Object.defineProperty, it's best not to polyfill Array.prototype methods at all, as you can't make them non-enumerable.

Specifications

Specification	Status	Comment
ECMAScript 5.1 (ECMA-262) The definition of 'Array.prototype.reduce' in that specification.	Standard	Initial definition. Implemented in JavaScript 1.8.
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array.prototype.reduce' in that specification.	Standard
ECMAScript 2017 Draft (ECMA-262) The definition of 'Array.prototype.reduce' in that specification.	Draft

Browser compatibility

See also

• Array.prototype.reduceRight()